The optical and electronic properties of semiconductor materials are largely determined by the bandgap, i.e., the difference in energy between the valence band and the conduction band. The bandgap of semiconductor materials generally decreases with rising temperature. For radiation-emitting optoelectronic semiconductor chips such as LEDs or semiconductor lasers, for example, this can result in the wavelength of the emitted radiation increasing with rising operating temperature. This can alter the perceived color of the emitted radiation.
It is also known that the sensitivity of the human eye decreases at wavelengths above about 560 nm. Thus for optoelectronic semiconductor chips that emit at relatively large wavelengths, a temperature-induced increase in the emission wavelength can result in not only a change in the perceived colour but also in a reduction in the visually perceived brightness.